- thermal metering apparatus



Sept- 3, 1940. w. c. DQWNING. JR 2,213,339`

THERMAL METERING `APIFARATUS I Filed Aug'. 31, 195B Nom-mene Tfn/0512670125 offLfc non emmene/anco Patented Sept. 3, 1940 UNITED STATES PATENT OFFICE v 2,213,939 f I' THERMAL METERING APPARATUS william c. Downing'. Jr., springfield, n1., assignmto Lincoln Meter fCompany, Inc., Springfield, Ill.,acorporation of Illinois Application August 31, 193s, serial No. 227,658

My invention relates, generally, to electrical measuring devices and it has particular relation to indicating thermal demandfmeters in which electric energy is converted into heat -that causes a. mechanical deflection proportional vto a func-l ll ous indication. on a chart in a graphic'type of instrument. As long-as the temperature of the two coils is the same,'no motion of the shaft results. even though this temperature changes. since the tendencyl of each of the coils to expand with rising temperature just balances that of -lthe other. However, when the temperature of one coil is higher than that of the other. there results a deflection of the'pointer carried by the shaft or the pen carried thereby which is proportional v to the temperature dierence between the coils.

The pair of coils, in this manner, constitutes a thermometer which vmeasures a difference in temperature rather than an absolute temperature.

Each of the bimetallic coils is contained in an individual enclosure and one or both of these enclosures is arranged to -be electrically heated. Heat applied to an enclosure tends to raise the temperature of the enclosure and the bimetallic coil contained therein by an amount closely pro 35 portional to the rate at lwhich the heat is applied.

The temperature diil'erence between enclosures, as measured bythe bimetallic coils, is therefore closely proportional to the difference in the rates of heat input to the two enclosures, hereinafter `40 referred to simply as the power input differential.

Various means, well known in the prior art, are employed to'provide a power input diilerential which is proportional or closely proportional to a function of the quantityto be measured.. Forexample, in a'n instrument intended for measuring the current flow in a two wire circuit, only one enclosure is heated. The power input differential is therefore in this case simply the power input to the heated enclosure, "and is proportional to the square of the current measured. The heating of the enclosure is accomplished by means of a non-inductive resistor which is connected directly in the circuit if the expected current iiow is below a certain predetermined value and is indirectly connected in the circuit by means of a shunt or a transformer if the current iiow in the circuit is expected to exceed a predetermined value. f-

Another example is that of an instrument for 5 measuring the flow of power in alternating current circuits. In this case heat is applied to both power flow it is intended ,to measure. Means 10` for doing this are described in the prior art Iand 1 are used in commercial instruments.

The operation of a thermaldemand meter employing the two opposed bimetallic coils is based upon the following assumptions: f

(1) A constant relation between the power input,

differential and some function of the quan- (2) A constant relation between the power input dmerentiar and temperature differential. 20

(3) A constanta relation between the temperature differential and the mechanical deflection of the shaft resulting in the movement of the pointer across the scale or the; movement of the pen across the chart. 25 While the third assumption is correct and the conditions there stated are .met very closely in thermal demand meters employing two opposed bimetallic coils, the conditions recited in assump' tions 1 and 2 are only approximately attained. 30 For example, ambient temperature variations may change the conditions recited in assumption 1 due to change of s ome or all of the electrical circuit resistances and certain of the'conditions in assumption 2 may be changed due to change 35 in heat transfer rates.

Ordinarily two adjustments are provided for a thermal demand meter. One of these is the zero setting adjustment and the other is a deflection adjustment. Both of these adjustments are pro- 40 vided to permit-.initial calibration of the meter and to permit recalib'ration of the meter after it has been'in service.

The zero adjustment is provided by means of a light spiral hair' spring'A which may be manually 45 rotated to add to or subtract from the amount of deflection of the shaft the desired amount so that vwhen no current is passing through the heating element or elements, thepointer or pen will be at the zeroposition. The action of the zero adjust- 50 ment is independent Vof the positions of the pointer or pen. y

The full load adjustment.or the deflection adjustmentds such that it alters the reading of any point on the scale by an amount which is propor- 55 tional to'thefri-inter denecaon. It is provided'by means of ahelicaiftension spring which is secured at one 4,eridfto the pointer, the tension ,thereot being: adjustable at the other end.I rThe helical tensionispring is so varranged that, when the pointer is'in the'zero .position-no torque is exerted thereon. {At all'oth'er. positions of the pointer, thehelic'altensionsprlng exerts a torque which is approximately proportional to the deflection.r f

. As indicated above. there are certain departures froml theas'sumed conditions for perfect accuracy of` the instrument. O ne of the principal sources ofv erroris the change in ambient temperature which; as indicated above, changes; the

electrical 'circuit 'resistancesof the heatinxxlements and alsochanges the heat transier'rates.

shaft and pointer or pen carried thereby. The f bimetallic helix is connected. to operate through the helical tension spring and-.to varythe tension thereof accordancewith change in ambient temperature: .This Vexternal compensating means has not been'entirely satisiactory.

Accordingly, the principalA object of. my invention is to so construct a thermal demand meter of the type herein described that compensation for changes in ambient temperature are inherent therein.

Another objectof my invention is to so proportion the thermally responsive elements of a thermal demand-meter that the provision of an external compensating device for changes in ambient temperature is unnecessary.

A further object of .my invention is to employ a bimetallic element or elements in a thermal demand meter having variable temperature-deiiection ratios in 'order to automatically compensate forchanges in ambient temperature.

According to the-present invention, the foregoing objects are accomplished by making either or both of the bimetallic elements of such materials that instead of having ay constant temperaturedeilection ratio or flinearvcharacteristics, they will have .a variable temperature-deflection ratio or non-linear characteristics in 'order to compensate for variations resulting` from changes in ambient temperature. By'prop erly .proportioning Lthe compositions Yoie-theA alloys 0.making up the' bimetallic strips, it'isj possible to increase the tem.-

peraturej-jdenection ratio with rising temperature. I! desiredthebimetallicstrips may be overcompensatedztp take caredfiother factors which tend to make the -indicatio' o1, lthe instrument inaccurate. n l

For a more cc` rn`1 al eteunderstandingV oi thev nature and'scopefoi'.` my invention reference may be had to-thefllwing. detailed description taken en t invention Refer-r in conneetionwith the accompanying drawing in whichthei single .il'guije illustrates diagrammatically a thermal Yder na'ndmeter in which my presiseibdid# Y r f'tdth'e'adrawing, it willbe obser\ .'ed`thatfv a .shaft Ilis provided and it will be understood ,th`at it'may be mounted for rotation on' bearings (not shown) Iat its ends. The shaft I0 carries' a pointer II which is arranged to cooperate with assale I2 in order to give an indication of the quantity being measured. .It will be understood that a pen may be substituted for the hereinbefore.

the elements I3 and Il may be changed so that asnasso pointer I I for drawing a line on a chart to graphically record the variations in the quantity being measured.

In order to rotate the shaft III inox-der to proyduce the desired deflection of the pointer II, a

pair of oppositely wound bimetallic coils, shown.

generally at 'I3 ad'll, is provided. The coil Il e Vends I.I and? I8 are secured to the shalt I0. In

the meter construction shown in the drawing a single heating element I9 is provided in the enclosure around the driving element I3. The heating element I9 comprises a non-inductive resistor which is connected in conductor 20 of a pair of conductors 20 and 2|;V .The conductors 2l and 2I may be connected ineither a direct current or an alternating current circuit.

As previously indicated, the elements I3 and I4 are formed by Abimetallic coils. They comprise iiat strips of bimetal that are coiled as illustrated. The outer layer 22 of metal may be Invar, an iron alloy containing iron 63.8%, nickel.36%, and car- -bon 0.2%. The inner layer 23 may comprise vide some auxiliary external means .for compensating for variations In change in ambient temperature.

I have found that the compensation for changes in ambient temperature may be made inherent in the construction of the bimetallic elements I3 and I4 so that it is unnecessary to employ the external compensating means. One manner of effecting this desired result is to increase thenickel content of the outer layer 22 of the Invarffrom about 36% to about 40%. A bimetallic strip is then obtained which has an increasing temperature-deflection ratio with rising temperature which may be employed for inherently compensating for the errors which resuit due to variationsin the conditions arising under the assumptions (1) v and (2) set forth It desired. the characteristics of the devicev will; be slightly overcompen ted.

While it has been particularlypcin out how the elements yland Il can bechanged by changing the alloy composition ofthe outer layer 22,

it will be understood that the compositions of both the outerand inner layers, 22 and 23 may be altered to produce the non-linear character` istics as desired. It will also beV understood that bimetals, other than the particular bimetal herein disclosed, may be employed it they have the `desired compensating characteristics. For extion or shown in the accompanying drawing shall u be interpreted as illustrative and.not in a limiting sense. i

I claim as'my invention: A a 1. 'A thermal meter for an electric circuit comprising, in combination, a shaft, indicating means carried by said shaft, a pair of oppositely wound bimetallic coils each fastened ,tosaidvsh'aft at one end and stationarily securedv at -"the other end, atgleast one of said bimetallic coils being characterized by having anon-linear tempera- 0 ture deifiection ratio with change in temperature to automatically compensate for different yambient temperatures under normal load conditions, and means for heating at leastvone of said coil`s in accordance with a variable characteristic of the electric circuit.

2. A thermal meter fcr an electric circuit comprising, in combination, a shaft, indicating means carried by said shaft, a pair of oppositeiy wound bimetallic coils each lfastened to said shaft vat one end and stationaril'y secured at the other end, both of said bimetallic coils being characterized by having substantially identical non-linear temperature deflection ratios vwith change in temperature such that'the indication afforded by substantially unaffected by changes in ambient said indicatingl means under load conditions is 'I temperature over the normal range thereof, and

means for supplying heat' to at least one of. said coils in accordance with a variable characteristic of the electric circuit. x v

-3. A thermal meter for anA electric circuit comprising, in combination,v ashaft, indicating means carried by said shaft, a pair of oppositely woundbimetallic coils each fastened to said shaft at one end and stationarily secured at the Arother end, one of the layers of metal forming each coil being of brass and the other being of Invar having a nickel content of about 40% v- WILLIAM c. DowNING, Jn. 25 

